-added a '--cmd' option to rez-env, for running a command directly inside
a configured env. Previously the only way to do this was to pipe into the
env using the -s flag.

NOTE: This was done because rez-run currently has a bug, where killing the
rez-run process doesn't kill the command itself. rez-run remains unchanged,
for now, but you can use "rez-env --cmd" instead, and here the bug is
fixed.